Halve potatoes lengthwise, then slice crosswise into 1/8-inch slices. (Do not put potatoes in water after slicing.) Immediately line up in pan with slices overlapping, arranging in serving portions. Pour butter or margarine over potatoes. Season with salt and pepper. Bake 20 minutes. Remove from oven; sprinkle with Swiss cheese and then with Parmesan cheese. Bake an additional 5-7 minutes or until cheese is melted and slightly browned.